Host cells comprising alpha 1, 2 mannosidase and culture methods thereof

ABSTRACT

Improved host cells and culture methods involving overexpression of MAN1C1 activity to improve protein production are provided.

The present application is a continuation of U.S. patent application Ser. No. 13/905,466, now allowed, which is a continuation of U.S. patent application Ser. No. 13/476,959, now issued as U.S. Pat. No. 8,460,896, which is a divisional of U.S. patent application Ser. No. 12/785,371, now issued as U.S. Pat. No. 8,247,210, which is a continuation of U.S. patent application Ser. No. 11/634,757, filed Dec. 6, 2006, now issued as U.S. Pat. No. 7,888,101, which claims benefit under 35 U.S.C. §119 of U.S. Patent Application No. 60/749,076, which was filed Dec. 8, 2005, each of which are hereby incorporated herein by reference in their entirety.

SEQUENCE LISTING

The present application is being filed along with a Sequence Listing in text format. The Sequence Listing is provided as a file entitled A-1072-US-CNT3_RevisedSeqListingfromDiv.TXT created Oct. 16, 2012, which is 27 KB in size. The information in the text format of the Sequence Listing is incorporated herein by reference in its entirety.

FIELD OF THE INVENTION

The invention relates to improved host cells and culture methods that improve glycoprotein production.

BACKGROUND OF THE INVENTION

Methods of increasing recombinant host protein production in the pharmaceutical industry and in the laboratory are highly desirable in many ways, including cost savings, times savings, and manufacturing capacity. Treatment with sodium butyrate has been one means of increasing protein production in cell culture in commercial biopharmaceutical processes. However, the derived benefit of increased protein yields is sometimes offset by the toxic side effects of sodium butyrate.

Sodium butyrate is a short chain fatty acid that inhibits the histone deacetylase (HDAC) enzyme responsible for the maintenance of chromatin structure in the nucleus of cells (Davie, J. Nutrition 133: 2485S-2493S, 2003). The loss of activity results in an alteration in transcriptional regulation of genes through the normal acetylation and deacetylation process of histones (Prasad et al., In Vitro 12: 125-132, 1976). The change in transcriptional regulation has been shown to increase the specific productivity of cell lines producing recombinant proteins in vitro. For example, sodium butyrate has been shown to increase the synthesis of secreted recombinant follicle stimulating hormone (FSH), tissue plasminogen activator (tPA), erythropoietin (EPO), and thrombopoietin (TPO) in Chinese hamster ovary (CHO) cells (Sung et al., J. Biotechnology 112: 323-335, 2004; Hendrick et al. Cytotechnology 36: 71-83, 2001; Chung et al., J. Microbiol. Biotechnol. 11, 1087-1092, 2001; Chotigeat et al., Cytotechnology 15: 217-221, 1994; Chang et al., Free Radical Research 30: 85-91, 1999). The precise mechanisms responsible for these increases are uncertain. Sodium butyrate treatment has been shown to transiently increase mRNA levels for recombinant protein, resulting in increases in resulting protein biosynthesis (Yuan et al., J. Biol. Chem. 260: 3778-3783, 1985).

Changes in gene expression caused by sodium butyrate have been studied previously in cell lines involved in colon cancer research. The studies demonstrated that sodium butyrate alters the expression of multiple genes involved in cell cycle progression, differentiation, cytokine signaling, and apoptosis. However, such studies were limited to relatively a small subset of genes and did not determine whether genes were involved in protein biosynthesis. (Joseph et al., Oncogene 23: 6304-6315, 2004; Tabuchi et al., Biochem. Biophys. Research Comm. 293: 1287-1294, 2002; Iacomino et al., Biochem. Biophys. Research Comm. 285: 1280-1289, 2001; Mariadason et al., Cancer Res. 60: 4561-4572, 2000; Della Ragione et al., FEBS Letters 499, 199-204, 2001).

Alpha 1,2 mannosidase I enzyme (MAN1C1) is an enzyme involved in glycoprotein N-linked oligosaccharide processing that has been described in Tremblay et al. (Glycobiology 8: 585-595, 1998) and Gonzalez et al. (J. Biol. Chem. 274: 21375-21386, 1999). The enzyme catalyzes the first mannose trimming step associated with processing of high mannose oligosaccharide structures by removing a terminal mannose sugar from the oligosaccharide. N-terminal glycosylation involves the addition and removal of various monosaccharide sugars in both the endoplasmic reticulum (ER) and Golgi compartments (Kornfeld et al., Ann. Rev. Biochem. 54: 631-664, 1985). In the ER, the N-linked glycosylation is accompanied by the folding of nascent glycoproteins into their native structure through interactions with molecular chaperones (Ellgaard et al., Science 286: 1882-1888, 1999; Jakob et al., J. Cell Biol. 142: 1223-1233, 1998). This process has been termed ER quality control, and if the process is blocked due to a misfolded protein, the onset of ER associated degradation, or ERAD, of the protein typically occurs (Ellgaard et al., Curr. Opin. Cell Biol. 13: 431-437, 2001; Sifers, Science 299: 1330-1331, 2003; Oda et al., Science 299: 1394-1397, 2003; Molinari et al., Science 299:1397-1400, 2003; Hurtley et al., Ann. Rev. Cell Biol. 5: 277-307, 1989). The removal of a terminal mannose sugar from Man₉ to Man₈ by the alpha 1,2 mannosidase I enzyme (MAN1C1) has been shown to affect the onset of the ERAD response (Liu et al., J. Biol. Chem. 274: 5861-5867, 1999; Grinna et al. J. Biol. Chem. 255, 2255-2258, 1980).

In view of the toxicity of protein production inducers such as sodium butyrate, there exists a need for other means of increasing overall production of recombinant proteins in cell culture.

SUMMARY OF THE INVENTION

The present invention provides host cells engineered to overexpress alpha 1,2 mannosidase (MAN1C1) and a glycoprotein or protein of interest. Such host cells may comprise a heterologous expression control sequence operably linked to a nucleic acid encoding MAN1C1, and a heterologous expression control sequence operably linked to a nucleic acid encoding such glycoprotein. The host cells may engineered to overexpress either or both MAN1C1 and the glycoprotein of interest by any means known in the art, including transfection with a vector comprising a nucleic acid encoding the protein, wherein the nucleic acid is operably linked to a heterologous expression control sequence, or transfection with an expression control sequence that upregulates expression of endogenous protein.

The invention further provides methods of producing a glycoprotein of interest comprising the steps of: culturing any of the host cells of the invention in culture medium; and recovering such glycoprotein from the host cell or culture medium. Such a host cell is cultured under conditions that induce increased MAN1C1 protein expression or increased specific productivity of the glycoprotein of interest. It is further contemplated that such a host cell expresses MAN1C1 protein at a level that increases the amount (pg/mg protein) or specific productivity (pg/cell/day) of such glycoprotein produced. Improvements in such glycoprotein production or specific productivity may be, e.g., at least 2-fold, 3-, 4-, 5-, 6-, 7-, 8-, 9-, 10-, 15-, or 20-fold or higher.

The invention also contemplates methods of increasing production of a glycoprotein of interest comprising the step of further adding an inducer of protein production to the culture medium. Inducers of protein production are well known to one of skill in the art and are further provided herein.

The host cells and methods of the invention may be used to produce any glycoprotein or protein of interest. Exemplary glycoproteins include erythropoiesis-stimulating molecules such as erythropoietin or darbepoetin, or analogs, variants, or derivatives thereof. Host cells of the invention may be, but are not limited to, mammalian cells, CHO cells, human cells, BHK cells, NS/0 cells, HT-1080 cells, or any other cell known to be useful in one of skill in the art.

In another aspect, the invention provides methods of screening for an inducer of protein production comprising the steps of: contacting a host cell with a candidate compound, determining expression level of MAN1C1, and identifying said candidate compound as an inducer of protein production if the expression level of MAN1C1 increases. Such a method contemplates determining mRNA expression levels or protein expression levels.

Other features and advantages of the invention will become apparent from the following detailed description. It should be understood, however, that the detailed description and the specific examples, while indicating exemplary embodiments of the invention, are given by way of illustration only, because various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 displays measured rHuEPO specific productivity (Q_(P)) over 5-day culture in the presence of sodium butyrate (SB) and absence (control) of 2 mM sodium butyrate.

FIG. 2A shows the effect of MAN1C1 siRNA treatment on rHuEPO Q_(P) in the presence of sodium butyrate_(SB) of and absence (control) of sodium butyrate over a 5-day period. FIG. 2B displays the fold change in MAN1C1 mRNA levels in HT1080 cells treated with sodium butyrate (SB) or sodium butyrate plus siRNA (SB+siRNA) as compared to control HT1080 mRNA levels.

FIG. 3 displays changes in rHuEPO mRNA levels in HT1080 cells treated with sodium butyrate in the presence (SB +siRNA) and absence of siRNA (SB) directed against MAN1C1 (n=3).

FIG. 4 displays changes in rHuEPO specific productivity after transient transfection with varying amounts of plasmid MAN1C1 DNA (n=2).

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides materials and methods for increasing the recombinant production of a protein of interest in host cells. As used herein, a “protein of interest” is a protein (other than MAN1C1) for which the recombinant production of bulk quantities of such protein is desired.

More specifically, the invention includes host cells producing a recombinant protein of interest that have been additionally engineered to overexpress alpha 1,2 mannosidase I enzyme (MAN1C1), cell cultures containing such host cells, and methods of producing increased amounts of the recombinant protein of interest comprising culturing such host cells under conditions such that MAN1C1 is expressed at levels higher than normal. The expression of higher levels of MAN1C1 results in improvements in specific productivity and/or protein production of the protein of interest.

The invention also contemplates methods of improving protein production or increasing specific productivity involving increasing the levels of MAN1C1 activity in host cells through introduction of chemical inducers that increase MAN1C1 protein production, or chemical inducers that increase the specific activity of MAN1C1 expressed.

The invention contemplates methods wherein the improved specific productivity measures at least 2 pg glycoprotein/cell/day. In exemplary embodiments, the specific productivity measures at least 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, or 100 pg glycoprotein/cell/day. However, greater specific productivity is contemplated, especially with the use of additional inducers.

The present invention determined that one mechanism by which sodium butyrate increases protein production is through upregulating expression of MAN1C1, a gene involved in glycosylation of proteins. The data described herein show that high levels of MAN1C1 expression do not significantly alter glycosylation of glycoproteins but unexpectedly increase the amount of recombinant protein produced. MAN1C1 mRNA expression was shown to dramatically increase when cells were treated with sodium butyrate, to roughly 10-fold higher over a 24 hour period and greater than 40-fold over five day period. Treatment of host cells expressing a recombinant protein of interest with siRNA to reduce MAN1C1 protein expression reduced by 50% the sodium butyrate-induced increase in production of the protein of interest. Moreover, overexpression of MAN1C1 in the absence of sodium butyrate resulted in a 2-3 fold increase in production of the protein of interest. Thus, the increased expression of MAN1C1 contributes to the increase in specific productivity of the protein of interest.

The term “isolated nucleic acid” refers to a nucleic acid of the invention that is free from at least one contaminating nucleic acid with which it is naturally associated. A “nucleic acid” refers to a DNA or RNA sequence, optionally including artificial bases or base analogs.

The term “identity” (or “percent identical”) is a measure of the percent of identical matches between the smaller of two or more sequences with gap alignments (if any) addressed by a particular mathematical model or computer program (i.e., “algorithms”). The term “similarity” is a related concept but, in contrast to “identity”, includes both identical matches and conservative substitution matches. Identity and similarity of related nucleic acid molecules and polypeptides can be readily calculated by known methods. Preferred methods to determine identity and/or similarity are designed to give the largest match between the sequences tested. Methods to determine identity and similarity are described in publicly available computer programs. Exemplary computer program methods to determine identity and similarity between two sequences include, but are not limited to, the GCG program package, including GAP (Devereux et al., Nucl. Acids. Res. 12: 387, 1984; Genetics Computer Group, University of Wisconsin, Madison, Wis.), BLASTP, BLASTN, and FASTA (Altschul et al., J. Mol. Biol. 215:403-410, 1990)). The BLASTX program is publicly available from the National Center for Biotechnology Information (NCBI) and other sources (BLAST Manual, Altschul et al. NCB/NLM/NIH Bethesda, Md. 20894; Altschul et al., supra). The well-known Smith-Waterman algorithm may also be used to determine identity. Preferred parameters for a polypeptide sequence comparison include the following: Algorithm: Needleman et al., J. Mol. Biol. 48: 443-453, 1970; Comparison matrix: BLOSUM 62 from Henikoff et al., Proc. Natl. Acad. Sci. USA 89: 10915-10919, 1992); Gap Penalty: 12, Gap Length Penalty: 4; Threshold of Similarity: 0. The GAP program is useful with the above parameters (along with no penalty for end gaps). Preferred parameters for nucleic acid molecule sequence comparisons include the following: Algorithm: Needleman et al., J. Mol. Biol., 48: 443-453, 1970; Comparison matrix: matches=+10, mismatch=0, Gap Penalty: 50, Gap Length Penalty: 3. The GAP program is also useful with the above parameters. Other exemplary algorithms, gap opening penalties, gap extension penalties, comparison matrices, thresholds of similarity, etc. may be used by those of skill in the art.

The term “operably linked” refers to a functional linkage between an expression control sequence and a second nucleic acid sequence, wherein the expression control sequence directs transcription of the nucleic acid corresponding to the second sequence.

The phrase “stringent hybridization conditions” refers to conditions under which a probe will hybridize to its target subsequence, typically in a complex mixture of nucleic acid, but to no other sequences. Hybridization stringency is principally determined by temperature, ionic strength, and the concentration of denaturing agents such as formamide. Examples of “highly stringent conditions” for hybridization and washing are 0.015M sodium chloride, 0.0015M sodium citrate at 65-68° C. or 0.015M sodium chloride, 0.0015M sodium citrate, and 50% formamide at 42° C. See Sambrook, Fritsch & Maniatis, Molecular Cloning: A Laboratory Manual, 2nd Ed., Cold Spring Harbor Laboratory, (Cold Spring Harbor, N.Y. 1989);(1989) and Anderson et al., Nucleic Acid Hybridisation:Hybridization: a practical approach, Ch. 4, IRL Press Limited (Oxford, England).Limited, Oxford, England (1999). Examples of typical “moderately stringent” conditions are 0.015M sodium chloride, 0.0015M sodium citrate at 50-65° C. or 0.015M sodium chloride, 0.0015M sodium citrate, and 20% formamide at 37-50° C. By way of example, a “moderately stringent” condition of 50° C. in 0.015 M sodium ion will allow about a 21% mismatch.

It will be appreciated by those skilled in the art that there is no absolute distinction between “highly” and “moderately” stringent conditions. For example, at 0.015M sodium ion (no formamide), the melting temperature of perfectly matched long DNA is about 71° C. With a wash at 65° C. (at the same ionic strength), this would allow for approximately a 6% mismatch. To capture more distantly related sequences, one skilled in the art can simply lower the temperature or raise the ionic strength.

The term “vector” is used to refer to any molecule (e.g., nucleic acid, plasmid, or virus) used to transfer coding information to a host cell.

The term “expression vector” refers to a vector which is suitable for use in a host cell and contains nucleic acid sequences which direct and/or control the expression of desired nucleic acid sequences (“expression control sequences”). Suitable expression control sequences include constitutive or inducible or regulatable promoters, enhancers or an array of transcription factor binding sites. “Expression” includes, but is not limited to, processes leading to protein production such as transcription, translation, and RNA splicing, if introns are present.

A “heterologous” expression control sequence operably linked to a nucleic acid refers to an expression control sequence that is operably linked to a nucleic acid (including a gene) that is different from the gene to which the expression control sequence is normally operably linked in its native state.

The term “host cell” is used to refer to a cell which has been transformed, or is capable of being transformed with a nucleic acid sequence and then of expressing a selected gene of interest. The term includes the progeny of the parent cell, whether or not the progeny is identical in morphology or in genetic make-up to the original parent, so long as the selected gene is present.

As used herein, a host cell “engineered to overexpress” a protein (or a nucleic acid encoding such protein) is a host cell, including a descendant thereof, that has been altered in such a way that higher levels of such protein are expressed than normal, compared to the unaltered host cell. Thus, included within this category are expression of proteins foreign to the host cell, proteins not naturally expressed by the host cell, or proteins naturally expressed by the host cell at relatively low levels that increase after alteration of the host cell.

The section headings are used herein for organizational purposes only, and are not to be construed as in any way limiting the subject matter described.

Increasing Alpha 1,2-Mannosidase (MAN1C1) Activity

The invention contemplates that increasing the levels of alpha 1,2-mannosidase (MAN1C1) activity in host cells during production of a protein of interest may be accomplished through any means known in the art, including by adding a chemical inducer or through overexpression of MAN1C1 enzyme. The MAN1C1 enzyme that the host cell overexpresses may have the same or similar sequence as a MAN1C1 that is endogenous, or native, to the host cell. Thus, human MAN1C1 may be overexpressed in human host cells, CHO MAN1C1 may be overexpressed in CHO host cells, and similarly native enzyme that carries out the MAN1C1 function may be overexpressed in other host cells. However, any MAN1C1 enzyme that functions in the same way in the desired host cell may be used, including an ortholog, or a biologically active fragment, variant, analog or derivative.

As used herein, “analog” refers to a nucleotide or amino acid sequence that has insertions, deletions or substitutions relative to the parent sequence, while still substantially maintaining the biological activity of the parent sequence, as determined using biological assays known to one of skill in the art. “Variants” include naturally occurring allelic variants, splice variants, or polymorphic forms of the parent sequence. “Derivatives” of naturally occurring, variant or analog polypeptides include those which have been chemically modified, for example, to attach water soluble polymers (e.g., polyethylene glycol), radionuclides, or other diagnostic or targeting or therapeutic moieties, any of which can be attached directly or indirectly through linkers.

“Biologically active” with respect to a MAN1C1 polypeptide means that the fragment, variant, derivative or analog thereof retains similar activity in improving specific productivity, e.g. as measured as amount of protein of interest produced per cell per day, or retains similar enzymatic activity in removing mannose from a suitable mannose-containing substrate. “Biologically active” with respect to a MAN1C1 nucleic acid means that the nucleic acid encodes such a biologically active MAN1C1 polypeptide.

The nucleotide and amino acid sequences of an exemplary human MAN1C1 are set forth in SEQ ID NOS: 1 and 2, respectively. Nucleotides 331 through 2223 of the polynucleotide of SEQ ID NO: 1 encode the MAN1C1 polypeptide of SEQ ID NO: 2. The nucleotide and polypeptide sequences for human MAN1C1 (SEQ ID NOS: 1 and 2, respectively, were identified by Tremblay et al. (J. Biol. Chem. 275: 31655-31660, 2000) and are provided in Genbank Accession No. AF261655. Exemplary polynucleotide and polypeptide sequences of other orthologs and variants of MAN1C1 are identified in Genbank Accession Nos. AB209275 (SEQ ID NOS: 12 and 13) and DV567987 (SEQ ID NO: 14).

The term “MAN1C1” as used herein refers to human MAN1C1 (the polypeptide of SEQ ID NO: 2 encoded by the polynucleotide of SEQ. ID NO: 1), orthologs thereof, or a biologically active fragment, variant, analog, or derivative of the human enzyme or orthologs. Exemplary analogs retain 65% or higher amino acid identity to the parent sequence, or 70%, 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, or 99% or higher identity. Exemplary fragments include fragments of at least 25, 50, 75, 100, or more amino acid residues of a MANIC1 polypeptide. Other exemplary MAN1C1 fragments, variants, analogs or derivatives include those encoded by nucleic acids that would hybridize under highly or moderately stringent conditions to the nucleotide sequence of SEQ ID NO: 1 or any other orthologs of the nucleotide sequence of SEQ ID NO: 1.

As used herein, the term “MANIC1 nucleic acid” or “MANIC1 polynucleotide” refers to a nucleic acid that encodes any of the preceding polypeptides, including a nucleotide sequence as set forth in SEQ ID NO: 1, or nucleic acids comprising nucleotide sequences that are at least about 65%, 70%, 75%, 80%, 85%, 90%, 95%, 96%, 97%, 98%, 99% or 100% identical thereto, or nucleic acids which hybridize under moderately or highly stringent conditions as defined herein with the complement of SEQ ID NO: 1 or any other orthologs of the nucleotide sequence of SEQ ID NO: 1.

It is also understood that MAN1C1 nucleic acids include allelic or splice variants of a MANIC1 nucleic acid of SEQ ID NO: 1 or any other orthologs, and include nucleotide sequences which are complementary to any of the above nucleotide sequences.

Where a gene encoding a MANIC1 polypeptide has been identified from one species, all or a portion of that gene may be used as a probe or primer to identify corresponding genes from other species (orthologs, or “species homologs”) or related genes from the same species (homologs). The probes or primers may be used for hybridization screening or PCR amplification of genomic DNA or cDNA libraries from various tissue sources believed to express the MANIC1 gene. Appropriate conditions of hybridization stringency can be determined by one of ordinary skill in the art. Bioinformatic techniques can also be used to identify orthologs, wherein collections of sequences from various mammalian or other species are screened for nucleotide or polypeptide sequences that exhibit significant homology to known MAN1C1 sequences. Nucleic acids encoding MAN1C1 polypeptides may also be identified by expression cloning which employs the detection of positive clones based upon a property of the expressed protein. Typically, nucleic acid libraries are screened by the binding of an antibody or other binding partner (e.g., receptor or ligand) to cloned proteins which are expressed and displayed on a host cell surface. The antibody or binding partner is modified with a detectable label to identify those cells expressing the desired clone.

Although the invention primarily contemplates that MAN1C1 will be overexpressed, i.e., expressed in the altered host cell at a greater level than normal in the unaltered host cell, the invention also contemplates methods for decreasing or inhibiting the expression of MAN1C1 in cells, e.g. through administration of siRNA or antisense compounds.

Inducers of Protein Production

The invention also contemplates the use of other known inducers of protein production in combination with the host cells which overexpress MAN1C1 and another protein of interest, to further increase overall production of the protein of interest. Known inducers include, but are not limited to, the following compounds: N-Acetyl-L-cysteine, Actinomycin D, 7-Amino-, Bafilamycin A1, Streptomyces griseus, Calphostin C, Cladosporium cladosporioides, Camptothecin, Camptotheca acuminata, CAPE, 2-Chloro-2′-deoxyadenosine, 2-Chloro-2′-deoxyadenosine 5′-Triphosphate, Tetralithium Salt, Cycloheximide, Cyclophosphamide Monohydrate, Cyclosporine, Trichoderma polysporum, Daunorubicin, Hydrochloride, Dexamethasone, Doxorubicin, Hydrochloride, (−)-Epigallocatechin Gallate, Etoposide, Etoposide Phosphate, ET-18-OCH3, 5-Fluorouracil, H-7, Dihydrochloride, Genistein, 4-Hydroxynonenal, 4-Hydroxyphenylretinamide, Hydroxyurea, IL-1β Inhibitor, (±)—S-Nitroso-N-acetylpenicillamine, S-Nitrosoglutathione, Phorbol-12-myristate-13-acetate, Puromycin, Dihydrochloride, 1-Pyrrolidinecarbodithioic Acid, Ammonium Salt, Quercetin, Dihydrate, Rapamycin, Sodium Butyrate, Sodium 4-Phenylbutyrate, D-erythro-Sphingosine, N-Acetyl-, D-erythro-Sphingosine, N-Octanoyl-, Staurosporine, Streptomyces sp., Sulindac, Thapsigargin, TRAIL, E. coli, Trichostatin A, Streptomyces sp., (±)-Verapamil, Hydrochloride, Veratridine, Vitamin D3, 1α, 25-Dihydroxy-, and Vitamin E Succinate (VWR and Calbiochem).

The invention further contemplates the identification of other chemicals that improve protein production through increasing MAN1C1 activity, for example via increasing MAN1C1 expression. Increases in MAN1C1 activity can be determined as described by Tremblay et al. (2000, supra).

Increases in MAN1C1 expression can be determined by measuring relative amounts of MAN1C1 mRNA produced as described in Example 4 (using Affymetrix chip) or Example 5 (quantitative PCR), or MAN1C1 protein produced via ELISA, HPLC, or other methods known in the art or as described by Tremblay et al. (2000, supra).

Proteins of Interest for Recombinant Production

The recombinant protein of interest that the host cell overexpresses can be any polypeptide, either endogenous (native) or exogenous to the cell. Exemplary proteins of interest are glycoproteins, including secreted glycoproteins. In exemplary embodiments, the glycoprotein of interest is an erythropoiesis-stimulating molecule, described below.

The amount of recombinant protein of interest produced may be measured as “specific productivity,” which is the amount of protein of interest produced per cell per day. The amount of recombinant protein of interest produced may also be measured by amount of protein of interest produced per amount of cell protein. Methods of measuring specific productivity or protein production are well known in the art.

The recombinant proteins of interest for which expression can be increased using the materials and methods of the invention can be any polypeptide, either endogenous or exogenous to the cell. Exemplary recombinant proteins of interest are glycoproteins, especially N-glycosylated glycoproteins. Exemplary glycoproteins of interest include secreted glycoproteins such as erythropoiesis-stimulating molecules.

The terms “polypeptide” and “protein” are used interchangeably herein.

The term “erythropoiesis-stimulating molecules” as used herein includes human erythropoietin (SEQ. ID NO.: 3) or a biologically active variant, derivative, or analog thereof, including a chemically modified derivative of such protein or analog. Amino acids 1 through 165 of SEQ ID NO: 3 constitute the mature protein. Another exemplary erythropoiesis-stimulating molecule is darbepoetin (SEQ ID NO: 5) Amino acids 1 through 165 of SEQ. ID NO: 5 constitute the mature protein. Also contemplated are analogs of erythropoietin (SEQ ID NO.: 3) or darbepoetin (SEQ. ID NO: 5), with 65%, 70%, 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, or 99% identity to SEQ. ID NO: 3 or SEQ. ID NO: 5, respectively, and still retaining erythropoietic activity.

Exemplary sequences, manufacture, purification and use of recombinant human erythropoietin are described in a number of patent publications, including but not limited to Lin U.S. Pat. No. 4,703,008 and Lai et al. U.S. Pat. No. 4,667,016, each of which is incorporated herein by reference in its entirety. Darbepoetin is a hyperglycosylated erythropoietin analog having five changes in the amino acid sequence of rHuEPO which provide for two additional carbohydrate chains. More specifically, darbepoetin contains two additional N-linked carbohydrate chains at amino acid residues 30 and 88 of SEQ ID NO: 5. Exemplary sequences, manufacture, purification and use of darbepoetin and other erythropoietin analogs are described in a number of patent publications, including Strickland et al., 91/05867, Elliott et al., WO 95/05465, Egrie et al., WO 00/24893, and Egrie et al. WO 01/81405, each of which is incorporated herein by reference in its entirety. Derivatives of naturally occurring or analog polypeptides include those which have been chemically modified, for example, to attach water soluble polymers (e.g., pegylated), radionuclides, or other diagnostic or targeting or therapeutic moieties.

The term “erythropoietic activity” means activity to stimulate erythropoiesis as demonstrated in an in vivo assay, for example, the exhypoxic polycythermic mouse assay. See, e.g., Cotes and Bangham, Nature 191:1065 (1961).

Other polypeptides of interest for which recombinant production can be increased using the materials and methods of the invention include cytokines, immunoglobulin-like proteins, antibodies, and peptibodies, and analogs, variants, or derivatives of any of these proteins.

Exemplary proteins of interest include granulocyte-colony stimulating factor (GCSF), stem cell factor, leptin, hormones, cytokines, hematopoietic factors, growth factors, antiobesity factors, trophic factors, anti-inflammatory factors, receptors or soluble receptors, such as a soluble fragment of p80 TNF-R, enzymes, and Fc-fusions of any of the preceding. Other examples include insulin, gastrin, prolactin, adrenocorticotropic hormone (ACTH), thyroid stimulating hormone (TSH), luteinizing hormone (LH), follicle stimulating hormone (FSH), human chorionic gonadotropin (HCG), motilin, interferons (alpha, beta, gamma), interleukins (IL-1 to IL-12), tumor necrosis factor (TNF), tumor necrosis factor-binding protein (TNF-bp) or TNF receptor-1 or —II, brain derived neurotrophic factor (BDNF), glial derived neurotrophic factor (GDNF), neurotrophic factor 3 (NT3), fibroblast growth factors (FGF), neurotrophic growth factor (NGF), bone growth factors such as osteoprotegerin (OPG), insulin-like growth factors (IGFs), macrophage colony stimulating factor (M-CSF), granulocyte macrophage colony stimulating factor (GM-CSF), megakaryocyte derived growth factor (MGDF), keratinocyte growth factor (KGF), thrombopoietin, platelet-derived growth factor (PGDF), colony simulating growth factors (CSFs), bone morphogenetic protein (BMP), superoxide dismutase (SOD), tissue plasminogen activator (TPA), urokinase, streptokinase, or kallikrein, receptors or soluble receptors, enzymes, variants, derivatives, or analogs including Fc-fusions of any of these proteins.

As used herein, the term “antibody” includes fully assembled antibodies, monoclonal antibodies (including human, humanized or chimeric antibodies), polyclonal antibodies, multispecific antibodies (e.g., bispecific antibodies), Maxibody, and antibody fragments that can bind antigen (e.g., Fab′, F′(ab)₂, Fv, single chain antibodies, diabodies), comprising complementarity determining regions (CDRs) of the foregoing as long as they exhibit the desired biological activity.

Exemplary antibodies are Herceptin® (Trastuzumab), a recombinant DNA-derived humanized monoclonal antibody that selectively binds to the extracellular domain of the human epidermal growth factor receptor 2 (Her2) proto-oncogene; and Rituxan®(Rituximab), a genetically engineered chimeric murine/human monoclonal antibody directed against the CD20 antigen found on the surface of normal and malignant B lymphocytes. Other exemplary antibodies include Avastin® (bevacizumab), Bexxar® (Tositumomab), Campath® (Alemtuzumab), Erbitux® (Cetuximab), Humira® (Adalimumab), Raptiva® (efalizumab), Remicade® (Infliximab), ReoPro® (Abciximab), Simulect® (Basiliximab), Synagis® (Palivizumab), Xolair® (Omalizumab), Zenapax® (Daclizumab), Zevalin® (Ibritumomab Tiuxetan), or Mylotarg® (gemtuzumab ozogamicin), receptors or soluble receptors, enzymes, variants, derivatives, or analogs of any of these antibodies.

Peptibodies, molecules comprising an antibody Fc domain attached to at least one antigen-binding peptide, are generally described in PCT publication WO 00/24782, published May 4, 2000 Immunoglobulin-like proteins, members of the immunoglobulin superfamily, contain one or more immunoglobulin-like domains which fold in structures similar to portions of the antibody variable region.

Engineering Host Cells to Overexpress Protein

Host cells can be engineered to overexpress a protein in a variety of ways known in the art, including but not limited to insertion of exogenous nucleic acid encoding the desired protein, optionally as part of an expression vector, insertion of an exogenous expression control sequence such that it causes increased expression of the host cell's endogenous gene encoding the desired protein, or activation of the host cell's endogenous expression control sequence(s) to increase expression of endogenous gene encoding the desired protein.

Cultures of host cells can be prepared according to any methods known in the art, and methods of growing such host cells and recovering recombinant protein produced by the cells, whether from the cells or culture medium, are known in the art. Such culturing methods may involve addition of chemical inducers of protein production to the culture medium. Exemplary host cells and procedures are described below.

A nucleic acid encoding a MAN1C1 polypeptide may be inserted into an appropriate expression vector using standard ligation techniques. Expression vectors optionally may include a promoter, one or more enhancer sequences, an origin of replication, a transcriptional termination sequence, a complete intron sequence containing a donor and acceptor splice site, a sequence encoding a leader or signal sequence for polypeptide secretion, a ribosome binding site, a polyadenylation sequence, a polylinker region for inserting the nucleic acid encoding the polypeptide to be expressed, and/or a selectable marker element. Each of these sequences is discussed below.

Optionally, the vector may contain a “tag”-encoding sequence, i.e., an oligonucleotide sequence located at the 5′ or 3′ end of the MAN1C1 polypeptide coding sequence; the oligonucleotide molecule encodes polyHis (such as hexaHis), or another “tag” such as FLAG, HA (hemaglutinin influenza virus) or myc for which commercially available antibodies exist. This tag is typically fused to the polypeptide upon expression of the polypeptide, and can serve as a means for detection or affinity purification of the MAN1C1 polypeptide from the host cell.

Suitable vectors include, but are not limited to, cosmids, plasmids, or modified viruses, but it will be appreciated that the vector system must be compatible with the selected host cell. Nucleic acid can be transferred into host cells by any means known in the art, e.g. through liposome-mediated transfer, receptor-mediated transfer (ligand-DNA complex), electroporation, microinjection of DNA, cell fusion, DEAE-dextran, calcium chloride, calcium phosphate precipitation, microparticle bombardment, infection with viral vectors, lipofection, transfection, or homologous recombination.

The invention also contemplates use of homologous recombination or other recombinant production methods utilizing control elements introduced into cells already containing DNA encoding MAN1C1 polypeptides. For example, homologous recombination methods may be used to modify a cell that contains a normally transcriptionally silent MAN1C1 gene, or an under expressed gene, and thereby produce a cell which expresses therapeutically efficacious amounts of MAN1C1 polypeptides. Homologous recombination is a technique originally developed for targeting genes to induce or correct mutations in transcriptionally active genes (Kucherlapati, Prog. Nucl. Acid Res. & Mol. Biol. 36: 301, 1989). The basic technique was developed as a method for introducing specific mutations into specific regions of the mammalian genome (Thomas et al., Cell 44: 419-428, 1986; Thomas et al., Cell 51:503-512, 1987; Doetschman et al., Proc. Natl. Acad. Sci. 85: 8583-8587, 1988) or to correct specific mutations within defective genes (Doetschman et al., Nature 330: 576-578, 1987). Exemplary homologous recombination techniques are described in U.S. Pat. No. 5,272,071 (EP 9193051, EP Publication No. 505500; PCT/US90/07642, International Publication No. WO 91/09955).

Through homologous recombination, the DNA sequence to be inserted into the genome can be directed to a specific region of the gene of interest by attaching it to targeting DNA. The targeting DNA is a nucleotide sequence that is complementary (homologous) to a region of the genomic DNA. Small pieces of targeting DNA that are complementary to a specific region of the genome are put in contact with the parental strand during the DNA replication process. It is a general property of DNA that has been inserted into a cell to hybridize, and therefore, recombine with other pieces of endogenous DNA through shared homologous regions. If this complementary strand is attached to an oligonucleotide that contains a mutation or a different sequence or an additional nucleotide, it too is incorporated into the newly synthesized strand as a result of the recombination. As a result of the proofreading function, it is possible for the new sequence of DNA to serve as the template. Thus, the transferred DNA is incorporated into the genome.

Attached to these pieces of targeting DNA are regions of DNA which may interact with or control the expression of a MAN1C1 polypeptide, e.g., flanking sequences. For example, a promoter and/or enhancer element, or an exogenous transcription modulatory element, optionally including an intron, is inserted in the genome of the intended host cell in proximity and orientation sufficient to influence the transcription of DNA encoding the desired MAN1C1 polypeptide. The control element controls a portion of the DNA present in the host cell genome. Thus, the expression of MAN1C1 polypeptide may be achieved not by transfection of DNA that encodes the MAN1C1 gene itself, but rather by the use of targeting DNA (containing regions of homology with the endogenous gene of interest) coupled with DNA regulatory segments that provide the endogenous gene sequence with recognizable signals for transcription of a MAN1C1 polypeptide.

In an exemplary embodiment, DNA which includes at least a regulatory sequence, an exon and a splice donor site is introduced into the chromosomal DNA in such a manner as to produce a new transcription unit (in which the regulatory sequence, the exon and the splice donor site present in the DNA construct are operatively linked to the endogenous gene).

Overexpression, as described herein, encompasses activating (or causing to be expressed) a gene which is normally silent (unexpressed) in the cell as obtained, as well as increasing the expression of a gene which is not expressed at physiologically significant levels in the cell as obtained.

Site-specific recombination systems such as Cre/loxP, FLP/FRT are known in the art (Sauer, Curr. Opin. Biotechnol. 521-527, 1994; Sauer, Meth. Enzymol. 225: 890-900, 1993).

An additional approach for increasing, or causing, the expression of MAN1C1 polypeptide from a cell's endogenous MAN1C1 gene involves increasing the expression of transcription factors that upregulate expression of the gene and/or decreasing the expression of transcriptional repressors that downregulate expression of the gene, in a manner which results in de novo or increased MAN1C1 polypeptide production from the cell's endogenous MAN1C1 gene.

Thus, the invention contemplates host cells into which nucleic acid encoding MAN1C1 has been inserted, optionally operably linked to an expression control sequence, and optionally as part of an expression vector. The invention also contemplates host cells into which a heterologous expression control sequence has been inserted in such a manner as to increase MAN1C1 expression, including host cells in which the native MAN1C1 gene is operably linked to a heterologous expression control sequence.

Any host cells or hosts known in the art for recombinant protein production may be used, including yeast cells, plant cells, plants, insect cells, and mammalian cells, and transgenic animals. Exemplary yeast cells include Pichia, e.g. P. pastoris, and Saccharomyces e.g. S. cerevisiae, as well as Schizosaccharomyces pombe, Kluyveromyces, K. Zactis, K. fragilis, K. bulgaricus, K. wickeramii, K. waltii, K. drosophilarum, K. thernotolerans, and K. marxianus; K. yarrowia; Trichoderma reesia, Neurospora crassa, Schwanniomyces, Schwanniomyces occidentalis, Neurospora, Penicillium, Totypocladium, Aspergillus, A. nidulans, A. niger, Hansenula, Candida, Kloeckera, Torulopsis, and Rhodotorula. Exemplary insect cells include Autographa californica and Spodoptera frugiperda, and Drosophila. Exemplary mammalian cells include varieties of CHO, BHK, HEK-293, NS0, YB2/3, SP2/0, and human cells such as PER-C6 or HT1080, as well as VERO, HeLa, COS, MDCK, NIH3T3, Jurkat, Saos, PC-12, HCT 116, L929, Ltk-, WI38, CV1, TM4, W138, Hep G2, MMT, a leukemic cell line, embryonic stem cell or fertilized egg cell.

Culturing Methods and Polypeptide Production

The invention also provides methods for culturing, i.e. growing, host cells under conditions that increase MAN1C1 protein expression and result in increased specific productivity or protein production of any of the recombinant proteins of interest described herein. Such methods may further include the step of recovering the recombinant protein of interest produced from the host cells or culture medium.

When the recombinant protein of interest is secreted into the medium, the medium can be harvested periodically, so that the same host cells can be used through several harvest cycles. In exemplary embodiments, host cells producing erythropoiesis-stimulating molecules are incubated in three discrete batch harvest cycles. For each cycle, medium is harvested and replaced with fresh medium replacing the harvested medium. The first cycle may be, e.g., 8 days; the second cycle, e.g., 7 days; and the third cycle, e.g., 5 days in duration.

A variety of culture systems are known in the art, including T-flasks, spinner and shaker flasks, roller bottles and stirred-tank bioreactors. Roller bottle cultivation is generally carried out by seeding cells into roller bottles that are partially filled (e.g., to 10-30% of capacity) with medium and slowly rotated, allowing cells to attach to the sides of the bottles and grow to confluency. The cell medium is harvested by decanting the supernatant, which is replaced with fresh medium. Anchorage-dependent cells can also be cultivated on microcarrier, e.g. polymeric spheres, that are maintained in suspension in stirred-tank bioreactors. Alternatively, cells can be grown in single-cell suspension.

Culture medium may be added in a batch process, e.g. where culture medium is added once to the cells in a single batch, or in a fed batch process in which small batches of culture medium are periodically added. Medium can be harvested at the end of culture or several times during culture. Continuously perfused production processes are also known in the art, and involve continuous feeding of fresh medium into the culture, while the same volume is continuously withdrawn from the reactor. Perfused cultures generally achieve higher cell densities than batch cultures and can be maintained for weeks or months with repeated harvests.

Host cells of the invention may be cultured using standard media well known to the skilled artisan. The media will usually contain all nutrients necessary for the growth and survival of the cells. Suitable media for culturing eukaryotic cells are, Roswell Park Memorial Institute medium 1640 (RPMI 1640), Minimal Essential Medium (MEM), and/or Dulbecco's Modified Eagle Medium (DMEM), all of which may be supplemented with serum and/or growth factors as indicated by the particular cell line being cultured. A suitable medium for insect cultures is Grace's medium supplemented with yeastolate, lactalbumin hydrolysate, and/or fetal calf serum as necessary.

Typically, an antibiotic or other compound useful for selective growth of transformed cells is added as a supplement to the media. The compound to be used will be dictated by the selectable marker element present on the plasmid with which the host cell was transformed. For example, where the selectable marker element is kanamycin resistance, the compound added to the culture medium will be kanamycin. Other compounds for selective growth include ampicillin, tetracycline, geneticin, and neomycin.

The amount of a MAN1C1 polypeptide and the amount of desired recombinant protein of interest produced by a host cell can be evaluated using standard methods known in the art. Such methods include, without limitation, Western blot analysis, SDS-polyacrylamide gel electrophoresis, non-denaturing gel electrophoresis, High Performance Liquid Chromatography (HPLC) separation, immunoprecipitation, and/or activity assays such as DNA binding gel shift assays. The invention also contemplates that specific productivity (expressed as pg/cell/day) of protein of interest can be evaluated using standard methods as known in the art and as described herein.

EXAMPLES

The present invention is described in more detail with reference to the following non-limiting examples, which are offered to more fully illustrate the invention, but are not to be construed as limiting the scope thereof. The examples illustrate various methods used in the invention, such as cell culture methods; recombinant HuEPO quantitation by RP-HPLC; analysis of gene expression using Affymetrix chips; quantitative real time PCR of selected genes; the use of MAN1C1 siRNA; and MAN1C1 cloning and expression. The examples also illustrate the effect of sodium butyrate on rHuEPO specific productivity and cell cycle progression; the effect of sodium butyrate on MAN1C1 expression; the effect of MAN1C1 small interfering RNA (siRNA) on rHuEPO specific productivity; and the effect of the overexpression of MAN1C1 on rHuEPO specific productivity.

The techniques described in these examples represent techniques described by the inventors to function well in the practice of the invention, and as such constitute exemplary modes for the practice thereof. Many changes can be made in the specific methods that are disclosed and still obtain a like or similar result without departing from the spirit and scope of the invention. Such variations are intended as aspects of the invention.

Example 1 Cell Culture Methods

A human kidney fibrosarcoma cell line, HT1080 (Rasheed et al., Cancer 33: 1027-1033, 1974), transfected with a plasmid for human EPO cDNA was used throughout the experiments described in the Examples as set out below. Selective pressure for the retention of the transfected plasmid was maintained with the antibiotic geneticin (Gibco). Cells were grown as attached monolayers in vented T-flasks (75 cm²) (Corning) at an inoculation density of 2.1×10⁴ cells/cm² in a humidified 12% CO₂ incubator at 37° C. Cells were grown for 4-5 days in 10% serum containing DMEM media supplemented with 1× Non-Essential amino acids, 5 mg/L geneticin, and 1.5 g/L sodium bicarbonate (all from Gibco). After cells reached confluence, they were washed with PBS to remove the serum and switched to serum-free 1:1 DMEM/F-12 media supplemented with 1× Non-Essential amino acids (Gibco), 1.5 g/L sodium bicarbonate, and 2 g/L glucose for a period of 5 to 7 days. Experimental cultures were treated with 2 mM sodium butyrate (Sigma) on the same day as the serum-free media addition.

Example 2 Recombinant HuEPO Quantitation by RP-HPLC

To determine the amount of recombinant human erythropoietin (rHuEPO) produced in cell culture media, 200 μl of media were analyzed by reversed phase HPLC. Samples were separated on an analytical styrene/divinylbenzene HPLC column equipped with a guard column (Polymer Labs) using a linear gradient from 30-65% CH₃CN in 0.1% TFA over 17 minutes (Sigma). The retention time for rHuEPO expressed in cell culture was approximately 15 5 minutes and corresponded with a purified rHuEPO standard (Amgen Inc). Integrated peak areas of unknown samples were quantitated by comparison to a standard curve of purified rHuEPO.

Example 3 Analysis of Gene Expression Using Affymetrix Chip

The expression of genes in HT1080 cells was studied over a 24 hour period in the presence and absence of sodium butyrate using oligonucleotide microarrays and the HU133A Affymetrix chip (Affymetrix). All experiments were carried out according to Affymetrix protocols. A total of 24 chips were analyzed—12 from sodium butyrate-treated cells and 12 controls. The gene expression data from the Affymetrix software was first imported into GeneSpring® version 6.0 and normalized using the software's global normalization procedure. Values below 0.01 were set to 0.01 and each measurement was divided by the 50^(th) percentile of all measurements for that sample. Each gene was then divided by the median of its measurements in all samples, and if the median of the raw values was below 10, then each measurement for that gene was divided by 10. Sample data for each chip was then grouped by treatment type (control or sodium butyrate) and by time (3, 6, 12, and 24 hours). Results from the expression data were compared to the publicly available gene list from the Consortium for functional Glycomics cDNA array (GLYCOv2 Gene Chip) for genes present in four categories: glycan degradation, glycan transferase, glycan transport, and sugar nucleotides, as defined by the Consortium gene list.

Example 4 Quantitative Real Time PCR of Selected Genes

TaqMan (Applied Biosystems Inc.) reverse transcription quantitative real time PCR (qRT-PCR) was used to verify the changes in mRNA expression seen using the Affymetrix chip. Probes to MAN1C1 and rHuEPO were generated using Primer Express software (Applied Biosystems Incorporated). MAN1C1: Forward—GGA GCC CCA GAG CCA AGT (SEQ ID NO: 6); Reverse—GCC AAG CAA ACT GCA TCA TCT (SEQ ID NO: 7); TaqMan—ECG AGC CCA GCG GGA GAA AAT CAX (E=6-FAM; X=Tamra) (SEQ ID NO: 8). rHuEPO: Forward—GTT AAT TTC TAT GCC TGG AAG AGG AT (SEQ ID NO: 9); Reverse—CCA GGC CCT GCC AGA CTT (SEQ ID NO: 10); TaqMan—EAG GTC GGG CAG CAG GCC GTX (E=6-FAM; X=Tamra) (SEQ ID NO: 11). An ABI 7000 or 7900 (Applied Biosystems Inc.) was used for each analysis with the following thermal cycling parameters; 1 cycle at 50° C. for 30 min; 1 cycle at 95° C. for 10 min; 40 cycles at 94° C. for 15 sec and 60° C. for 60 sec. RNA samples from at least three independent cell cultures were assayed in triplicate for both control and sodium butyrate-treated cells. The cycle threshold (Ct) values for all genes tested were normalized to the housekeeping gene, GAPDH (Applied Biosystems Inc., part #4310884E), to correct for error in RNA concentrations. Data was reported as either “fold change” or “percent change” in Ct levels for sodium butyrate treated samples as compared to control samples.

Example 5 MAN1C1 siRNA

HT 1080 cells were grown in 75 cm² vented T-flasks for 4 days in 10% serum containing DMEM media supplemented with 1× Non-Essential amino acids, and 1.5 g/L sodium bicarbonate (all from Gibco). After cells reached confluence they were washed lx with PBS and then treated with or without 50 μL siPORTT™ Amine transfection agent plus 60 μM of small inhibitory RNA (siRNA) (Ambion) in 15 mL of 10% serum containing DMEM media for a period of 48 hours. Cultures were then washed with PBS to remove serum and switched into 15 mL serum free DMEM:F12 media with or without 50 μL siPORT™ Amine and 60 uM siRNA. Cultures were then grown for a period of 5 days in the presence and absence of 2 mM sodium butyrate. Samples of media were collected and rHuEPO and mRNA levels were assayed on Days 1, 3, and 5 post serum-free shift.

Example 6 MAN1C1 Cloning and Expression

A pENTR™221 entry vector containing the cDNA sequence of MAN1C1 (SEQ ID NO: 1) and flanking attL recombination sites was used (Invitrogen—clone ID:10H42767). Following the gateway technology procedure, recombination, using LR Clonase™ of the entry clone two attL sites with the destination pcDNA3.1/nV5-DESTT™ vector two attR sites, was performed and verified by transformation of DHSα competent cells under ampicillin selection. (The destination vector contains the ccdB gene that allows for negative selection in the recombination reaction.) Restriction enzyme digestion of the destination vector after recombination and transformation with BamHI confirmed the proper recombination reaction (data not shown). A plasmid preparation of DNA (˜3 mg) was obtained using the manufacturer's protocol for the Giga Prep Plasmid DNA Kit (Qiagen).

Transfection of HT1080 cells was performed as follows: T-flasks (75 cm²) were inoculated at 1.6×10⁶ cells and allowed to grow for 4 to 5 days prior to transfection. Either 16, 32, or 64 μg of plasmid DNA was added to 2 mL of DMEM:F12 media. In addition, 59 μL Lipofectamine 2000 (Invitrogen) was added to a separate 2 mL of DMEM:F12 media and incubated for 5 minutes. The two mixtures of DNA and Lipofectamine were then combined and allowed to incubate for 20 minutes at room temperature. Following a 1× PBS wash of the T-flasks, the entire mixture (˜4 mL) was added and allowed to incubate for 2 hours in a 37° C./12% CO₂ incubator. An additional 11 mL of fresh DMEM:F12 media was added to each T-flask, and sampling began 24 hours later.

Example 7 Sodium Butyrate Increases rHuEPO Specific Productivity and Blocks Cell Cycle Progression

The effect that sodium butyrate treatment has on the HT1080 cell cycle and rHuEPO specific productivity was examined Cells treated with sodium butyrate (2 mM) increased recombinant human erythropoietin (rHuEPO) specific productivity roughly four-fold compared to control cultures (FIG. 1). However, instead of sodium butyrate shifting the cells into the G0/G1 phase, where other investigators have reported protein synthesis is maximized (Kim et al., Biotech. Bioeng. 71: 184, 2001), the population of cells in G0/G1, S, and G2/M-phases remained relatively constant over the entire 5 days of culture as compared to control cultures. These differences may be due to the culturing methods as set out above, which allowed the cells to reach confluence prior to the addition of sodium butyrate, whereas previous reports looked at cell cycle effects on exponentially growing cells at the time of sodium butyrate addition. Therefore, cell cycle data alone cannot be responsible for the increases in rHuEPO specific productivity that were measured.

Example 8 Sodium Butyrate Increases MAN1C1 Expression

Recombinant HuEPO, produced in HT1080 cultures, was used as a model glycoprotein to study the effects of sodium butyrate on genes involved in protein glycosylation. Treatment of HT1080 cells with 2 mM sodium butyrate in culture resulted in numerous phenotypic changes with respect to sugar nucleotide pools and the oligosaccharide structures present on rHuEPO.

To determine if these phenotypic changes were associated with genetic changes, the expression of genes in HT1080 cells was studied over a 24 hour period in the presence and absence of sodium butyrate using oligonucleotide microarrays and the HU133A Affymetrix chip (Affymetrix).

A potential rate-limiting enzyme, alpha 1,2 mannosidase I enzyme (MAN1C1), involved in glycoprotein N-linked oligosaccharide processing, was identified. The relative change in MAN1C1 mRNA induced by sodium butyrate is set forth below in Table 1. This gene increased in expression roughly 10-fold over the course of twenty-four hours when cells were treated with sodium butyrate (Table 1). Subsequent validation of the MAN1C1 expression by qRT-PCR for HT1080 cells treated with sodium butyrate over a five day period showed increases >40-fold as compared to control cultures (Table 1).

TABLE 1 Summary of qRT-PCR Data for HT1080 MAN1C1 mRNA Fold Change Time Point (hr)^(A) Fold Change^(B) Standard Deviation 3 8.7 ±2.1 6 38.7 ±9.5 12 36.0 ±4.6 24 53.3 ±14.4 72 42.2 ±3.8 120 7.8 ±3.6 ^(A)Time after sodium butyrate added to culture ^(B)Fold change as compared to untreated HT1080 cells

Example 9 MAN1C1 Small Interfering RNA (siRNA) Reduces rHuEPO Specific Productivity

To determine if a link existed between sodium butyrate's effect on increases in MAN1C1 expression and increases in rHuEPO specific productivity, cells were treated with small interfering RNA (siRNA) against MAN1C1 in the presence and absence of sodium butyrate. Treatment with siRNA had no impact on rHuEPO Q_(P) under control conditions, but decreased rHuEPO Q_(P) 50% when cells were treated with both sodium butyrate and the siRNA as compared to sodium butyrate alone (FIG. 2A). As previously shown, sodium butyrate treatment increases MAN1C1 mRNA levels as compared to control cultures (FIG. 2B), however, cells treated with both sodium butyrate and siRNA had an ˜30% decrease in MAN1C1 mRNA levels as compared to cells treated with sodium butyrate alone (FIG. 2B). It should be noted that MAN1C1 mRNA levels were still higher in the sodium butyrate and siRNA treated cells as compared to controls (FIG. 2B). This correlated to a rHuEPO Q_(P) that remained higher than control in both cells treated with sodium butyrate alone and cells treated with sodium butyrate and siRNA, although the absolute levels of Q_(P) were reduced (FIG. 2A).

To ensure that changes in specific productivity measurements were not caused by a decrease in rHuEPO mRNA levels due to off target effects of the siRNA treatment, rHuEPO mRNA levels were measured over the same five-day period. As shown in FIG. 3, rHuEPO mRNA levels were consistent on day 1 and day 5 in cells treated with sodium butyrate (+/−) siRNA, and were slightly higher on day 5 as compared to control cultures. This data confirms that the siRNA treatment for MAN1C1 did not alter rHuEPO expression levels by reducing rHuEPO mRNA abundance.

Example 10 Overexpression of MAN1C1 Increases Specific Productivity of rHuEPO

To confirm that MAN1C1 could account for the increase in rHuEPO Q_(P), independent from sodium butyrate treatment, cells were transfected to overexpress MAN1C1 under control conditions. As seen in Table 2 (set out below), when 16-64 μg of MAN1C1 plasmid DNA was transfected into cells, a large increase (>1000 fold) in MAN1C1 mRNA levels was measured as compared to un-transfected (control) cells.

The transient expression of MAN1C1 under control conditions (e.g. no sodium butyrate addition) resulted in a 2-3 fold increase in rHuEPO specific productivity depending on the amount of plasmid DNA transfected (FIG. 4). Although the level of rHuEPO productivity is less than when sodium butyrate alone is used, the data confirms that sodium butyrate increases MAN1C1 expression, which in turn contributes to the increase in rHuEPO specific productivity.

TABLE 2 Summary of % MAN1C1 mRNA increase after transfection as compared to untransfected cells. % MAN1C1 mRNA Increase (n = 2) ^(A) Sample Day 1 Day 3 Day 5 16 ug DNA 1683% 1817% 2020% 32 ug DNA 1776% 1828% 2149% 64 ug DNA 1650% 1774% 2463% ^(A) % increase was calculated by the difference in the cycle at which absolute quantitation occurs (Ct—cycle threshold) between cells untransfected versus transfected.

All publications, patents and patent applications cited in this specification are herein incorporated by reference in their entirety, including but not limited to the material relevant for the reason cited, as if each individual publication or patent application were specifically and individually indicated to be incorporated by reference. Although the foregoing invention has been described in some detail by way of illustration and example for purposes of clarity of understanding, it will be readily apparent to those of ordinary skill in the art in light of the teachings of this invention that certain changes and modifications may be made thereto without departing from the spirit or scope of the appended claims. 

We claim:
 1. A host cell engineered to overexpress (a) alpha 1,2 mannosidase (MAN1C1) native to the host cell and (b) a glycoprotein of interest.
 2. The host cell of claim 1 wherein said host cell has been transfected with a vector comprising a nucleic acid encoding MAN1C1 operably linked to a heterologous expression control sequence.
 3. The host cell of claim 1 wherein said host cell comprises a heterologous expression control sequence operably linked to a nucleic acid encoding MAN1C1.
 4. The host cell of any of claims 1-3 wherein said host cell has been transfected with a vector comprising a nucleic acid encoding said glycoprotein of interest operably linked to a heterologous expression control sequence.
 5. The host cell of any of claims 1-3 wherein said host cell comprises a heterologous expression control sequence operably linked to a nucleic acid encoding said glycoprotein of interest.
 6. The host cell any of claims 1-5 wherein the glycoprotein of interest is an erythropoiesis-stimulating molecule.
 7. The host cell of claim 6 wherein the glycoprotein of interest is erythropoietin of SEQ ID NO:
 3. 8. The host cell of claim 6 wherein the glycoprotein of interest is darbepoetin of SEQ ID NO:
 5. 9. The host cell of any of claims 1-8 which is a mammalian cell.
 10. The host cell of any of claims 1-8 which is a CHO cell.
 11. The host cell of any of claims 1-8 which is a human cell.
 12. The host cell of any of claims 1-8 which is a BHK cell.
 13. The host cell of any of claims 1-8 which is an NS/0 cell.
 14. The host cell of any of claims 1-8 which is an HT-1080 cell.
 15. A method of producing a glycoprotein of interest comprising the steps of: culturing the host cell of any of claims 1-14 in culture medium; and recovering the glycoprotein of interest from the host cell or culture medium.
 16. The method of claim 15 wherein said host is cultured under conditions that induce increased MAN1C1 expression.
 17. The method of claim 15 wherein said host cell expresses MAN1C1 protein at a level that increases specific productivity (pg/cell/day) of glycoprotein of interest produced.
 18. The method of claim 17 wherein at least a 2-fold increase in specific productivity is achieved by overexpression of MAN1C1.
 19. A method of increasing production of a glycoprotein of interest comprising the steps of: culturing the host cell of any of claims 1-14 in culture medium under conditions that permit expression of MAN1C1 at levels that improve production of the glycoprotein of interest; and recovering the glycoprotein of interest from the host cell or culture medium.
 20. The method of claim 15 comprising the step of further adding an inducer of protein production to the culture medium.
 21. A method of screening for an inducer of protein production comprising the steps of: (a) contacting a host cell with a candidate compound, (b) determining expression level of MAN1C1, and (c) identifying said candidate compound as an inducer of protein production if the expression level of MAN1C1 increases.
 22. The method of claim 21 wherein MAN1C1 mRNA expression level is determined.
 23. The method of claim 21 wherein MAN1C1 protein expression level is determined. 